Why Would You Nerf Nunu Even a Little Bit?

The current change on the PBE:

W- Blood Boil Duration: 12 seconds -> 7 seconds Cool Down: 15 seconds -> 10 seconds Mana Cost: 50 mana -> 35 mana Movement Speed Given: 8/9/10/11/12% -> 10%

This may seem like a neutral change, but it actually isn't where it counts. Allow me to explain why.

1. Mana Costs on Nunu are not a huge thing. Just basic attack 5 times and the mana cost no longer matters. The mana cost buff is very minimal for Nunu's strength.
2. The cool down and duration buffs may seem like they equal out, but what is actually happening is Nunu needs more CDR to obtain a permanent W. Previously, Nunu would only need 20% CDR, reducing his cool down to 12 seconds, an amount that equals the duration. Now Nunu needs 30% CDR for a permanent W. While not huge, it is still a nerf.
3. Chances are, the main Nunu play style that makes use of his W is Support Nunu. While this may buff his early game slightly, the 2% movement speed nerf impacts Support Nunu for a much larger time throughout the game since Support Nunu maxes W first or second, depending on the game. Plus, percentage movement speed bonuses matter a lot more later in the game since just a little bit of movement speed later in the game can be the difference between a multi-kill and no multi-kill. So, I ask again, why are we nerfing Nunu? He is not strong at all right now.